(1) Field of the Invention
The present invention relates to the general technical field of flight control systems, in particular for aircraft and more generally for aircraft such as airplanes, rotorcraft, or helicopters.
The present invention relates more particularly to flight control systems suitable for implementing autopilot functions. Such control systems serve to adjust attitude-varying moving members of an aircraft such as an airplane or a rotorcraft as a function of the actuation of control members acting on at least one flight control linkage.
(2) Description of Related Art
Flight control systems are known for controlling a swashplate of a helicopter main rotor. In such an example, the moving members are constituted by hydraulic servo-controls that are connected to the swashplate of the main rotor. Action on a cyclic stick, e.g. action taken by the pilot, acts on the angle of inclination of the swashplate about two perpendicular axes. The angle of inclination of the swashplate then imparts an angle of inclination to the attitude of the main rotor, which angle depends on the angular movement of the cyclic stick.
In known manner, moving the cyclic stick from front towards the rear or vice versa causes the aircraft to move in pitching, while moving the cyclic stick from right to left or vice versa causes the aircraft to perform a roll movement. The pitching and roll movements of the aircraft are thus generated by imparting corresponding tilt to the main rotor.
The roll and pitching flight control axes are preferably embodied by distinct control linkages connecting the attitude-varying moving members to the cyclic stick.
The flight control linkages of a helicopter may be fitted with stabilizer actuators, referred to as series actuators, that are controlled by an autopilot system. These series actuators are connected in series in the flight control linkages and they move the attitude-varying moving members situated downstream from the positions of said series actuators. Nevertheless, the movements of the moving members must not give rise to movements of other members situated in the flight control linkages and situated upstream from the positions of said series actuators, e.g. they must not move the cyclic sticks.
Certain autopilot systems include parallel actuators, referred to as trim actuators, that are installed in parallel with the flight control linkages as close as possible to the sticks and/or the pedals, and that serve automatically to correct the positions of the moving members. It is in general such trim actuators that serve to anchor the stabilizer actuators.
In a simplified autopilot system that does not include trim actuators and that merely provides stabilization on two axes, the pitching and roll axes, it is possible to use a friction mechanism on the two cyclic control axes as a replacement for anchoring the stabilizer actuators.
By way of example, document FR 764 635 describes friction means for cyclic flight control. The friction means comprise two independent friction mechanisms on two distinct control axes. Each friction mechanism includes an adjustment wheel. The adjustment wheel may be tightened to a greater or lesser extent against a slideway and serves to define the magnitude of the friction in each friction mechanism in independent manner. The use of two friction mechanisms increases the number of on-board parts and weight, thereby constituting a drawback that is not negligible for an aircraft of the helicopter type. Furthermore, those friction means are generally set once and forever during initial assembly or subsequent revision, and they are then no longer accessible from the cockpit.
Document FR 2 678 577 mentions an embodiment in which the friction means use a hemispherical ball joint at the base of the cyclic stick. The ball joint is used for pivoting the cyclic stick and serves to constitute friction means. The ball joint has a stationary friction cup secured to the floor of the aircraft, with the stick passing therethrough, together with a friction cup connected to said stick. The friction cup bears against the stationary cup. A knob surrounding the stick makes it possible to adjust the level of friction between the two cups. The knob serves to modify friction simultaneously about both control axes (roll and pitching). In spite of that simplified adjustment, those friction means present a certain number of drawback. Those friction means are mechanically complex and often unreliable or not accurately reproducible in terms of setting levels. The adjustment of the friction means is also difficult for the copilot to access.
Document FR 2 295 473 also discloses an autopilot system including series actuators with amplitude-limited authority. A flight director system gives the pilot information enabling the pilot to recenter the series actuator when they are close to their extreme extension or retraction positions. A preadjusted friction member is provided in an embodiment in order to keep the control stick in the position to which it was put by the pilot.
When the series actuators are installed relatively close to the cyclic stick, known embodiments present the major drawback that the preadjusted level of friction needed for performing the function of anchoring said actuators constitutes an impediment for the pilot when it is necessary to take back control of the aircraft.